1. Field of the Invention
The present invention relates to a spin transistor and magnetic memory, e.g., a spin transistor having output characteristics depending on the spin direction of carriers, and a magnetic memory capable of recording information by using the spin transistor.
2. Description of the Related Art
Recently, new devices using the electric charge of electrons and the properties of spins at the same time are extensively researched. A spin transistor (also called a spin-transfer torque transistor or spin-filter transistor) as one of these devices is a transistor using a magnetic material as the source electrode and drain electrode, and capable of controlling the output characteristics by changing the relative magnetization directions in the source electrode and drain electrode (e.g., S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84[2004]2307). That is, this transistor uses a phenomenon in which a drain current Id(P) increases when the relative magnetization directions in the source electrode and drain electrode are almost parallel, and a drain current Id(AP) decreases when the relative magnetization directions are almost antiparallel.
When using the spin transistor in a memory or reconfigurable logic circuit, it is necessary to increase the Id(P)/Id(AP) ratio, i.e., the difference between the drain current when the relative magnetization directions in the source electrode and drain electrode are almost parallel and that when the relative magnetization directions are almost antiparallel. To increase the Id(P)/Id(AP) ratio, it is necessary to, e.g., (1) increase the spin polarization ratio of the magnetic material used as the source electrode and drain electrode, (2) increase the efficiency of spin transfer to the semiconductor channel, or (3) suppress spin diffusion and spin relaxation in a semiconductor. Presently, spin diffusion and spin relaxation in a semiconductor are large, and this makes it impossible to secure a high Id(P)/Id(AP) ratio required for applications.
Note that it is also possible by adjusting the types of semiconductor and magnetic material to form a so-called inverse spin transistor in which the drain current Id(P) decreases when the relative magnetization directions in the source electrode and drain electrode are almost parallel, and the drain current Id(AP) increases when the relative magnetization directions are almost antiparallel. In this case, spin diffusion and spin relaxation in a semiconductor must be reduced in order to increase the Id(AP)/Id(P) ratio.